feat(desktop): add Edit menu with Select All command

Ensure standard Edit menu operations work with terminal:
- Copy (Cmd+C) - copies selected text to clipboard
- Paste (Cmd+V) - pastes from clipboard to terminal
- Select All (Cmd+A) - selects all terminal content

SwiftTerm handles copy/paste via NSPasteboard.general for
cross-application clipboard support.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: grokify

apps/desktop/Sources/PlexusOneDesktop/App/PlexusOneDesktopApp.swift

@@ -48,6 +48,17 @@ struct PlexusOneDesktopApp: App {
                 .keyboardShortcut("a", modifiers: [.command, .shift])
             }
 
+            // Edit menu - ensure copy/paste work with terminal
+            CommandGroup(after: .pasteboard) {
+                Divider()
+
+                Button("Select All") {
+                    // Forward to first responder (terminal view handles this)
+                    NSApp.sendAction(#selector(NSText.selectAll(_:)), to: nil, from: nil)
+                }
+                .keyboardShortcut("a", modifiers: .command)
+            }
+
             CommandGroup(after: .windowList) {
                 Divider()